1. Field of the Invention
The present invention relates to an information processing system for encryption/decryption of digital information, or in particular to an information processing system and an information processing method for supplying power through a serial bus such as a USB terminal.
2. Description of the Related Art
The recent progress of the digital technologies has promoted the development/manufacture and extended the use of the information processing system capable of encryption/decryption of digital information. Also, this digital information processing system has been reduced in size and has come to be widely used as a handy tool for handling digital information.
This information processing system often uses a USB (universal serial bus) terminal for communication with other information processing systems. The USB, which is based on the communication standard and has a power supply other than a signal line, facilitates the connection and disconnection with the system powered on. The USB power terminal, which is very convenient in terms of the communication standard, can also be used to supply power for the information processing system as well as the communication information.
As a conventional technique in this connection, Jpn. Pat. Appln. KOKAI Publication No. 2001-242965 discloses a PC (personal computer) for supplying power to a digital information device through the USB terminal. Thus, power can be supplied to the digital information device, for example, through the USB terminal.
In the conventional PC described above with power supplied to an information device through the USB terminal, however, it is impossible to supply only power to the information device and to perform the control operation by key operation on the part of the information device. Once the operation mode is changed to the PC mode, for example, the operation is possible only with a command issued from the PC. With the home AC power supply, therefore, the normal operation with the operation switch of the information device cannot be performed, which otherwise might be possible by supplying from the USB the DC power rectified by a power adaptor.
Specifically, in the case where power is supplied through the USB terminal from the power adaptor, the information device cannot be operated by the key operation in view of the fact that the same situation prevails as if power is supplied to control the operation by an application on the PC. As a result, the information device remains in standby state until a control signal is supplied to the USB terminal from an external PC. In the case where the control signal fails to be supplied, an error occurs or the standby state lasts, thereby posing the problem that power cannot be supplied from the power adaptor through the USB terminal. Further, in the case where power is supplied from the USB terminal, the control operation is possible only from an external PC, and therefore the control operation, even if desired, by both the operation key of the information device and the external PC cannot be performed.